A catalytic hydrocarbon fuel reformer converts a fuel stream comprising, for example, natural gas, light distillates, methanol, propane, naphtha, kerosene, gasoline, diesel fuel, or combinations thereof, and air, into a hydrogen-rich reformate fuel stream comprising a gaseous blend of hydrogen, carbon monoxide and nitrogen (ignoring trace components). In the reforming process, the raw hydrocarbon fuel stream is typically percolated with oxygen in the form of air through a catalyst bed or beds contained within reactor tubes mounted in the reformer vessel. The catalytic conversion process is typically carried out at elevated catalyst temperatures in the range of about 700° C. to about 1100° C.
The produced hydrogen-rich reformate stream may be used, for example, as the fuel gas stream feeding the anode of an electrochemical fuel cell after passing the reformate stream through a water gas shift reactor and/or other purification means such as a carbon monoxide selective oxidizer. Reformate is particularly well suited to fueling a solid oxide fuel cell (SOFC) system because the purification step for removal of carbon monoxide is not required for an SOFC.
During operation of most reformers of this type, tail gas from the fuel cell is burned and the burner exhaust passes through a plenum within the vessel, contacting and heating the outer surface of the reactor tubes and thereby heating the catalyst.
The hydrogen-rich reformate stream may also be used as a hydrogen fuel to fuel a spark-ignited (SI) engine, either alone or in combination with gasoline. Hydrogen-fueled vehicles are of interest as low-emissions vehicles because hydrogen as a fuel or a fuel additive can significantly reduce air pollution and can be produced from a variety of fuels. Hydrogen permits an engine to run with very lean fuel-air mixtures that greatly reduce production of NOx. As a gasoline additive, small amounts of supplemental hydrogen fuel may allow conventional gasoline internal combustion engines to reach nearly zero emissions levels.
A problem in the past has been how to elevate the temperature of the catalyst quickly in order to begin generating reformate in the shortest possible time. An approach disclosed in the parent to this application is to incorporate into the reformer a “fast light-off” system wherein a fuel/air mixture, essentially stoichiometric, is ignited in the reformer, preferably upstream of the catalyst, for a brief period at start-up. The exhaust gas, passing through the reformer in contact with the catalyst, heats the catalyst very rapidly. Such combustion typically is needed for only a few seconds, after which ignition is terminated and the mixture is made very fuel-rich for reforming.
A problem exists, however, in how to make the transition from the fuel-lean mixture and combustion to the fuel-rich mixture and reforming. It is desirable to extinguish combustion before changing over from lean to rich mixture to prevent brief but intense coking of the catalyst surfaces caused by burning the rich mixture. However, simply stopping ignition has been found to be insufficient. One approach has been to include a flame arrestor between the combustion chamber and the reformer, which approach can be successful in preventing coking but also has been found to reduce very substantially and undesirably the rate at which the pre-combustion heats the catalyst, thus extending undesirably the start-up period.
What is needed is a means for changing over from combustion mode to reforming mode very quickly without a flame arrestor and also without coking of the catalyst surfaces.